Custom Search

Thevenin’s theorem

Practically always occurs that a particular element like load is always variable when the other elements are fixed in a circuit. If we consider our household as example a household outlet terminal may be connected to different appliances constituting a variable load. Every time the variable is changed, the complete circuit has to be analyzed again. This is harassment for electrical engineer. To avoid this problem we can use the method of Thevenin’s theorem. The fixed circuit is replaced by an equivalent circuit and according to this method.

thevenins theorem

Figure 1(a) shows a linear circuit connected with a variable load, the load can be single resistor or different circuit.

thevenins theory

According to Thevenin’s theorem we can replace figure 1(a) linear circuit with figure 1(b). Left side of the figure 1(b) is said Thevenin’s equivalent circuit. French telegraph engineer M.Leon Thevenin developed it in 1883.

Thevenin’s theorem states that “ a linear two terminal circuit can be replaced by an equivalent circuit which have a voltage source VTh in series with a resistor RTh where VTh is the open circuit voltage at the terminals and RTh is the input or equivalent resistance at the terminals when the independent sources are turned off””.

We have to find the Thevenin equivalent voltage VTh and resistance RTh. For doing this let us consider figure 1(a) & figure 1(b) are equivalent. The two circuits will be equal if they have same voltage- current relationship at their two terminals.

Let us find that what will make two circuits are equivalent. If we remove the load from figure 1(a), the voltage source will be open circuit then voltage source of figure 1(a) must equal to voltage source VTh in figure 1(b). So the two circuits will become equivalent and open circuit voltage across two terminals is

VTh = Voc

And figure 2(a) shows that

thevenins equivalent circuit

If the load is disconnected a-b terminals are open circuit. Now we turn off the all independent source the input equivalent resistance of figure 1(a) is equal to input resistance RTh and figure 2(b) shows it and that is

RTh = Rin

thevenin's equivalent circuit

For applying this method we have to find out Thevenin’s resistance RTh considering two cases.

  • Case 1: If the network has no dependent sources, we turn off all independent sources. RTh is the input resistance of the network looking between terminals and b as shown in figure 2(b).
  • Case 2: If the network has dependent sources, we turn off all independent sources. As with superposition theorem, dependent sources are not to be turned off because they are controlled by circuit variables.

Applying a voltage source v0 at terminals a and b to determine resulting current i0 .

RTh = v0 / i0as shown in figure 3(a). We can apply a current source alternatively at a and b terminal as shown in figure 3(b).

thevenins theorem with voltage source

thevenins theorem with current source

The two processes will give same result. In either process we suppose any value of v0 and i0. As example we can use v0 = 1V or i0 = 1A or even can use unspecified values of v0 or i0.


The value of RTh takes negative always. In this situation the negative resistance (v = – iR) values of RTh tells that the circuit is supplying power.


Importance of Thevenin’s theorem:


In circuit analysis Thevenin’s theorem is very important. It is helpful simplifying a circuit. A large circuit can be replaced with an independent voltage source and a resistor. This replacement system is very helpful tool in circuit design.


According to Thevenin’s theorem we replace a huge circuit with Thevenin’s equivalent, actually original circuit behaves same as Thevenin’s equivalent.


Consider a linear two terminal circuit shown in figure 4(a).

thevenins equivalent ckt

Load current IL and voltage across the load vL can be easily determined if the Thevenin’s equivalent circuit at the load terminals is obtained shown in figure 4(b).

thevenins equivalent

From figure 4(b) we get

 thevenins equivalent equation

Above equation shows Thevenin’s equivalent is a simple voltage divider, yielding VL by mere inspection.

Thevenin’s theorem problem:

math of thevenin's theorem

Find the Thevenin’s equivalent circuit of the circuit shown in figure 5, to the left of the terminals a-b. Then find the current through RL = 6, 16, 36Ω.


Answer: Turning off voltage source and making short circuit and 2A current circuit replacing it open circuit we get the circuit shown in figure 6 (a)

thevenins theorem mathematic

To find VTh applying mesh analysis consider figure 6(b).

thevenins theorem math

In two loops applying mesh analysis we get,


-32 + 4i1 + 12 (i1 – i2) = 0, i2 = -2A


i1 = 0.5A.


In this way VTh = 12 (i1 – i2) = 30v


We can use alternative way nodal analysis or source transformation to find VTh.


The Thevenin’s equivalent circuit shown in figure 7.

thevenins theorem maths

We know

IL = VTh / (RTh + RL) = 30 / (4 + RL)

When RL = 6,   IL = 30/10 = 3A

When RL = 16, IL = 30/20 = 1.5A

When RL = 36, IL = 30/40 = 0.75A



Share this Story
  • Thevenin’s theorem

    Practically always occurs that a particular element like load is always variable when the other elements are fixed in a circuit. ...
Load More Related Articles
  • Force on a Current Carrying Conductor in a Magnetic Field

    We know moving charge experiences a force due to magnetic field. Magnetic field creates force into conductor passing current through it. ...
  • Wheatstone Bridge

    Wheatstone bridge is a bridge where four resistances are used. First and second resistance are connected in series; third and fourth ...
  • Biot Savart Law

    To understand completely Biot Savart Law some important rules or laws should be known are Maxwell cork screw Law, Fleming’s right ...
  • Electrical Shunt

    Galvanometer is very sensitive device. Flow of huge amount current can burn galvanometer or displaces spring of it. For saving galvanometer ...
  • 1st Law of Thermodynamics

    Scientist Rumford and Joule did experiment in thermodynamics. In 1847 Joule showed relation between heat and mechanical energy. He explained from ...
  • Reversible process in thermodynamics

    The process is called reversible process which can run back reversely with the same quantity of heat goes forward process. Forward ...
Load More By Md Ebrahim Shah
  • Voltage divider rule

    Voltage divides in series circuit only. In parallel circuit voltage remains same. All of the loads are connected parallelly across voltage ...
  • Kirchhoff’s law

    Kirchhoff gave two laws for current and voltage. First law is called KCL and second law is called KVL. Kirchhoff’s current ...
  • Norton’s theorem

    American engineer Norton explained his theorem in 1926 after 43 years publishing Thevenin’s theorem. Norton’s theorem is similar to Thevenin’s theorem. ...
  • Thevenin’s theorem

    Practically always occurs that a particular element like load is always variable when the other elements are fixed in a circuit. ...
  • Inductor

    An inductor is a passive two terminal element designed to store energy in its magnetic field. Inductor also called reactor or ...
  • What is capacitor?

    A capacitor is an element which stores charges. It is a passive element as it stores energy. Michael Faraday invented capacitor. ...
Load More In dc circuits

Check Also

Force on a Current Carrying Conductor in a Magnetic Field

We know moving charge experiences a force due ...

© Copyright 2009-2017, All Rights Reserved.